Internal combustion engines generate drive torque by combusting an air and fuel mixture within cylinders. Exhaust that is generated via the combustion process is exhausted from the cylinders and is treated in an after-treatment system. During the combustion process, fuel is oxidized and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds.
The after-treatment systems traditionally include a catalytic converter that reduces exhaust emissions by chemically converting the exhaust gas into carbon dioxide (CO2) nitrogen (N), and water (H2O). In some cases, a lean NOx catalyst is implemented. Lean NOx technology, also known as HC selective catalytic reduction (SCR) has various formulations (e.g., platinum/alumina, copper and substituted zeolite. Platinum on alumina (Pt/Al2O3) functions at law temperatures has higher peak conversion of approximately 40% at 225° C., but has a very narrow temperature window of operation (e.g., between 180-280° C.). As a result, this formulation is not very useful by itself. Another disadvantage of platinum catalysts has been their SOx oxidation activity and if s susceptibility to deactivation by sulfur.
NOx absorbers have also been developed based on acid-base wash-coat chemistry. The NOx is absorbed and is stored in the NOx absorber catalyst wash-coat during lean operating conditions (i.e., higher than stoichiometric air to fuel ratio). The NOx is released and is catalytically converted to nitrogen during rich operating conditions (i.e., lower than stoichiometric air to fuel ratio). Barium-based NOx absorbers have high conversion efficiency but are only active at increased temperatures (e.g., greater than approximately 250° C.). Also, NOx absorbers require periodic desulfation.